It was hard enough remaking Clash of the Titans as a state-of-the-art ode to Ray Harryhausen without having to contend with a very compressed post schedule and then a last-minute 3-D conversion. But somehow it all worked out, according to Nick Davis, the overall visual effects supervisor, and Chris Bond, president of Prime Focus, which did the conversion of the Louis Letterier-directed epic.

"We basically had 23 weeks to do 900 very hard visual effects shots," Davis admits about the Greek fantasy involving Perseus (Sam Worthington), son of Zeus (Liam Neeson). "It was a real push because originally it was going to be a summer movie and then it was decided we'd come out in March [and finally April 2], so to lose three months is a big deal. It puts huge amounts of pressures on everybody to start the process so much earlier, which is a difficult thing to do when you're in the heat of battle shooting and trying to craft a movie."

Those 900 vfx shots were divided primarily between the London houses MPC (supervised by Gary Brozenich), Cinesite (supervised by Simon Stanley-Clamp) and Framestore (supervised by Tim Webber).

Davis worked with Aaron Sims on crucial designs of the Harpies, Scorpiochs and the Kraken. The Scorpiochs sequence was handled by Cinesite: "That went through a huge process, not only designing the articulation of the creature, but also the rigging, the size and scale," Davis continues. "And it raised a lot of interesting problems: How much like a scorpion should it look like? And it very much came from the studio that they didn't want Land of the Giants, so we went down this reptilian look where it has an armored shell around itself, but also it was quite lithe and could move quickly. We did a lot of animation studies with it and obviously prevised the sequence [Nvizage]. We found the location in the Canary Islands on Tenerife Park. We wanted to shoot the whole thing on location so it had a real grounded, lit feel to it. We took all of our interactive parts in a four-axis motion base, built by Neil Corbould [the special effects supervisor] and his crew. We used a mixture of pre-animated movements and on the spot, freestyle, control of the rig, and from that we got some great results as one of our characters, Draco [Mads Mikkelsen] leaps onto the back of the Scorpiochs and is flung around in all directions. And then the Scorpiochs wound up getting bigger and bigger and bigger until the final one shows up."

The Harpies, meanwhile, are in two sequences, including the climax where they chase Perseus riding on Pegasus through the streets of Argos on his way to battling the Kraken. MPC handled the Harpies, Pegasus, Argos and the Kraken. "We had a lot of interaction and had to deal with the Pegasus at that point," Davis adds. "Sam was on various rigs and we had stuntmen that we did head replacements on. Obviously every time we saw the real horse, we had to track the CG wings onto it. It was quite a challenge tracking something on that's moving and all black. MPC tracked the wings onto every shot and also did the fully CG Pegasus whenever it was flying."

The Kraken, of course, was a tremendous five-month undertaking, not the least of which was trying to avoid what ILM had achieved with the mythological squid monster in Pirates of the Caribbean: Dead Man's Chest.

"Once we got the design down, then it was a matter of figuring out how this 500-foot creature would look and behave," Davis suggests. "How could we have it wallowing in a bay and not look so stupid that we'll never work again? We wanted tentacles, but didn't completely want to anthropomorphize it and have a bloke wandering through, so we wanted to have the feeling that it was from the depths of the sea. That was something MPC worked very hard on, developing their water pipelines and software using FlowLine and creating multiple splashes and water simulations that enabled us to get the feeling of thousands of gallons being flicked through the air and the impact of it colliding with other water surfaces and the creature constantly pouring out of every crest like a moving water fall. And that took a lot of development and render time. We had this 45-second shot of the Kraken rising up out of the Bay of Argos and they used about 260 water simulation passes on that to create the different caches that would make up this enormous volume of water cascading off the different parts of the creature, but it got shortened in the movie. And, of course, every time it impacts another surface that required a different FlowLine system to make those collisions work."

For the ancient city of Argos, MPC created all of the geometry for the various buildings using photographic textures from India, Morocco and Europe for photorealistic grounding.

"For this city, which is built on 2,000-foot cliffs, we went to this place called Los Gigantes in Tenerife, and we took thousands of digital stills of the cliffs and they became the basic diorama through which the city was found," Davis adds. "And we built it at different resolutions knowing that certain areas had to be a lot more detailed than others."

The fabled Medusa was created by Framestore, with the biggest decision being how to animate the 50-odd snakes in her hair. Considering the short post schedule, they thought the best solution was keyframe animation. Even so, it was very time-consuming and difficult because of "the collisions and colliding and crossing over within the snakes." Actually, she was based on Russian supermodel Natalia Vodianova. "We did a photographic texture shoot of her, we did body and facial scanning, some basic performance capture with her and then, from that, she became the basic model that we created Medusa from," Davis explains. "And then on top of that, we wanted there to be a change within her when she goes into her petrified state, so we built a secondary model of her face that becomes a lot more demonic with her brows being pushed and her eyes blazing and her teeth becoming snake teeth and the entire texture of her face changing into more reptilian scales."

For Mount Olympus, Framestore was pulled into a completely different design. "One of the challenges of Olympus is that we shot on a set that in post-production everyone decided that they weren't that keen on anymore," Davis remarks. "We came up with a concept of the floor being a map of the world with moving clouds on it. This makes the Gods glow and it's much more of a surreal environment. We took the artists off the original set and set them in a new environment and actually rotoscoped every single shot. Framestore had to redo hundreds of shots in a few months."

And what was Davis' reaction to Warner Bros.' last-minute decision to go 3-D? "We were in shock. Here we were getting near the end of post-production, where we were in the thick of it with reshoots and changes, and, suddenly, we had this announcement that we were gonna do it 3-D [in less than 10 weeks]. So it was a bit of a challenge thrown everybody's way. But I have to say it's been a very educational process. I've been involved for the last couple of weeks, working here [in L.A.] with Prime Focus and this post stereoscopic conversion definitely has its place and has a big future."

Still, there's a lot riding on this conversion, given the industry's sudden appetite for everything 3-D. Yet despite all the carping about Clash sight unseen and post conversion in general, Prime Focus has an opportunity to distinguish itself with its proprietary View-D process.

"We really attempted to keep the original cinematography intact in every way that we could," Bond emphasizes. "They wanted depth, but not in your face gimmickry, and didn't want to further miniaturize creatures or environments.

"In Clash, actors have fly-away hairs and tassels on their clothing, and what I've seen with other conversion techniques is a tendency to cut these things out. But we've kept all those pieces so you'll have a foreground actor [with fine detail]."

With View-D, Bond says there is an underlying depth generation system, which is very fast and offers lots of iterations. "There's not a huge calculation time and it's very deterministic, meaning that it's consistently repeatable, so that on this film, in order to work on it in the timeframe we had, we worked on scanned plates and received graded plates later and ran them through the same tools and got the same results. So we were able to work with different pieces of material during different stages of the post-production process and not lose a lot of time. And there were days when we saw 19 minutes of footage. But normally we got to the point where we were handling 10-12 minutes of iterations a day. It's a very efficient process and we hope to make that even more streamlined."

Clash was split up into three phases: selection, depth generation (conversion) and convergence adjustment. Selection, which determines what elements are worked on in 3-D space, was spread out among Prime Focus' offices in Vancouver, Winnipeg, L.A. and India. In fact, the bulk of this phase was done in India, with a peak of 200 staffers, because of the extensive crowd sequences requiring roto work to maximize dimensionality. Depth generation, or the actual View-D step, was done completely in L.A., with a staff of around 40 making use of three stereo screening rooms, a Dolby Digital 3-D system and two RealD systems. Convergence adjustment was also done in L.A., with a couple of people overseeing shot finals plus 10 coordinators, a producer, a production manager and an editorial team of five.

"We'll have to leave it up to the audience to see whether they enjoy the experience," Davis concludes," but I have to say that I'm very impressed with the volume that they're able to create and the speed with which they've been able to do it."